1. Field of the Invention
This invention relates to a rubber composition having a good processability and capable of giving a vulcanizate having excellent properties. More particularly, it relates to a rubber composition comprising a diene copolymer, having a relatively low molecular weight, produced by copolymerizing chloromethylstyrene or a glycidyl group-containing monomer, and a cross-linking agent reactive with the glycidyl group or chloromethyl group in which the diene copolymer is difficult to migrate and accordingly the properties of its vulcanizate are not deteriorated.
2. Description of Prior Art
As a method for improving the processability of rubber, a softening agent such as an extender oil, a petroleum resin or the like has hitherto been blended therewith. However, in such a method for improving the processability of rubber by the use of such softening agents, there are problems that the properties of its vulcanizate are deteriorated, and the softening agent in the vulcanizate bleeds out to cause an insufficient adhesion to other vulcanized rubber compositions contacted with the vulcanizate, or the softening agent migrates into said other vulcanized rubber compositions contacted with the vulcanizate to adversely affect the properties of vulcanized rubber composition, even though the processability of rubber can be improved. Accordingly, it has recently been desired in many fields of rubber industry to develop softening agents which hardly migrate, namely, so-called reactive softening agents.
In order to suppress the deterioration of the vulcanizate properties due to softening agents, it is known to use a low molecular weight rubber as a softening agent for the sake of improving the processability and, at the same time, to co-vulcanize the low molecular weight polymer and the rubber. However, this co-vulcanization does not always progress sufficiently, so that the migration of the softening agent is not sufficiently inhibited, and the properties of vulcanizate are not sufficiently stable for a long period of time.
The present inventors have conducted extensive studies for obtaining a rubber composition having an improved processability, in which the low molecular weight polymer contained therein as a softening agent does not migrate even in its vulcanizate and hence, the properties of the vulcanizate do not deteriorate. As a result, it has been found that a low molecular weight copolymer containing glycidyl groups or chloromethyl groups acts as a softening agent for the rubber composition at the time of processing and said low molecular weight copolymer is converted into a product having a high molecular weight and a crosslinking structure by the reaction of the glycidyl groups or chloromethyl groups with the crosslinking agent upon vulcanization, so that the rubber composition containing said low molecular weight copolymer exhibits a good processability in the unvulcanized state and the properties of its vulcanizate are excellent.